The present invention relates to an apparatus for detecting misfiring in an internal combustion engine, and more particularly to a misfire detecting apparatus for an engine of a motor vehicle.
Misfiring is a phenomenon which occurs in an internal combustion engine when a cylinder of the engine fails to ignite. Misfiring can be due to a number of causes, such as a failure of the ignition system to generate an adequate spark in a cylinder or a failure of a fuel supply system to supply a proper air/fuel mixture to the cylinder. When misfiring is due to a failure of the ignition system, uncombusted fuel is discharged from a misfiring cylinder. The discharge of uncombusted fuel from an engine is of course undesirable because of possible damage to a catalytic converter of the engine and for other reasons. Therefore, it is desirable to be able to detect occurrence of misfiring in an engine and then cut off the supply of fuel to the cylinder in which misfiring has occurred.
Under the circumstances, a typical example of a known misfiring detecting apparatus is shown in FIG. 4. In this Figure, a pulse sensor 1 mounted on an internal combustion engine generates an angular pulse signal containing a series of pulses in synchronism with the rotation of a crankshaft of the engine. A speed sensor 2 receives the angular pulse signal output from the pulse sensor 1, senses angular velocities of the crankshaft at two (i.e., first and second) predetermined angular positions or crank angles thereof before and after top dead center of each cylinder, and generates corresponding speed signals 6, 7. A speed deviation calculator 3 receives the output signals 6, 7 from the speed sensor 2, calculates a deviation between the angular velocities at the first and second crank angles or positions, and generates a corresponding output signal 8 which is input to a misfiring determiner 4. Based on the output signal 8 from the speed deviation calculator 3, the misfiring determiner 4 determines whether the engine is misfiring. That is, the misfiring determiner 4 generates a misfiring signal 9 if the angular velocity deviation obtained by the speed deviation calculator 3 is less than a predetermined value.
In operation of the above-described misfiring detecting apparatus, if misfiring takes place in the engine, the angular velocity deviation obtained by the speed deviation calculator 3 will greatly decrease in comparison with that obtained in the case where normal combustion takes place in the engine. That is, such a great decrease in the angular velocity deviation is detected by the misfiring determiner 4, i.e., by comparing the angular velocity deviation with a predetermined reference value and determining the occurrence of misfiring if the angular velocity deviation is less than the reference value.
Instead of using two angular velocities at the two crank angles before and after top dead center of each cylinder, two angular velocities obtained at two predetermined angular positions or crank angles of the crankshaft both after top dead center of each cylinder may be employed for calculating a deviation therebetween. In this case, too, misfiring determination is performed in the same manner as described above.
With the above-mentioned known misfiring detecting apparatus, however, the angular velocity deviation decreases as the rotational speed of the engine increases or as the engine load decreases. In other words, when the rotational speed of the engine is in a high speed range, or when the engine load is in a low load range, or when the engine is in an engine starting period in which combustion is incomplete or insufficient, the angular velocity deviation calculated by the speed deviation calculator 3 becomes less than the predetermined reference value, so it is mistakenly determined that the engine is misfiring, notwithstanding the fact that normal combustion is taking place in the engine.